In the 3rd Generation Partnership Project (3GPP), standardization of techniques for improving deterioration of communication quality due to recent sharp increase in mobile traffic and for achieving faster communication has been performed. Further, standardization of techniques for avoiding an increase in a control signaling load due to connections of an enormous number of Machine to Machine (M2M) terminals to a Long Term Evolution (LTE) or LTE-Advanced network has been performed. The M2M terminals are, for example, terminals that perform communication without human intervention. The M2M terminals are placed in various types of equipment including machines (e.g., vending machines, gas meters, electric meters, vehicles, railway vehicles, and ships) and sensors (e.g., environmental, agricultural, and traffic sensors). In the LTE and LTE-Advanced, communications performed by the M2M terminals is referred to as Machine Type Communication (MTC) and a terminal performing the MTC is referred to as an MTC terminal (i.e., MTC User Equipment (MTC UE)).
While M2M service providers need to distribute an enormous number of M2M terminals, there is a limit to the cost allowable for each M2M terminal. Therefore, it is required that M2M terminals be implemented at a low cost, and M2M terminals be able to perform communication with low power consumption, for example. Further, in one use case, MTC UEs perform communication while they are fixedly or statically installed in buildings. In this case, the radio quality of the MTC UEs may be always low and accordingly coverage enhancement technique is especially needed for the MTC UEs compared to normal UEs having mobility (e.g., mobile telephones, smartphones, tablet computers, and notebook personal computers (notebook PCs)). Further, functional restrictions contributing to reduction of the cost include, for example, a low maximum transmission power, a small number of reception antennas (e.g., only one reception antenna), no support of high-order modulation schemes (e.g., 64 quadrature amplitude modulation (64QAM)), and a narrow operating bandwidth (e.g., 1.4 MHz), which lower the maximum transmission rate of the MTC UEs.
Therefore, in the 3GPP, standardization of techniques for improving or enhancing communication characteristics of MTC UEs (i.e., coverage), which are expected to be lower than those of normal UEs, has been performed (Non-Patent Literature 1). The following description provides some examples of the techniques for enhancing coverage of MTC UEs discussed in the 3GPP. It can be said that the coverage enhancement techniques (or coverage enhancement processing) for MTC UEs described below are processing for improving or enhancing communication characteristics or communication quality of MTC UEs. The state of a UE to which these special coverage enhancement techniques have been applied is referred to as a Coverage Enhancement (CE) Mode, a Coverage Extension (CE) Mode, an Enhanced Coverage Mode (ECM), or an Extended Coverage Mode (ECM).
The coverage enhancement techniques can improve, for example, a reception characteristic of a Physical Broadcast Channel (PBCH), a transmission characteristic of a Physical Random Access Channel (PRACH) preamble (i.e., detection characteristic in a radio base station (an evolved NodeB (eNB))), a reception characteristic of a Physical Downlink Control Channel (PDCCH), a reception characteristic of a Physical Downlink Shared Channel (PDSCH), a transmission characteristic of a Physical Uplink Control Channel (PUCCH), and a transmission characteristic of a Physical Uplink Shared Channel (PUSCH). The PBCH is a downlink broadcast channel used by an eNB to transmit common broadcast information in a cell. The PRACH is an uplink physical channel used by a UE for an initial access (i.e., a random access) to an eNB. The PDCCH is a downlink physical channel used for, for example, scheduling information of downlink data (DL assignment) and transmission of radio resource allocation information of uplink data (UL grant) by an eNB. The PDSCH is a downlink physical channel used for reception of system information and data by a UE. The PUSCH is an uplink physical channel used for data transmission by a UE.
One processing that is being discussed to improve the reception characteristic of the PBCH is to repeatedly transmit broadcast information about the PBCH a number of extra times as compared to the normal operation by a certain number of times (see Non-Patent Literature 2). One processing that is being discussed to improve the transmission characteristic of the PRACH is to repeatedly transmit the PRACH (i.e., preamble) a certain number of times (see Non-Patent Literature 3). Further, one processing that is being discussed to improve the reception characteristic of the PDSCH and the transmission characteristic of the PUCCH and the PUSCH is to repeatedly transmit the PDSCH, the PUCCH, and the PUSCH over multiple subframes (see Non-Patent Literature 4). Further, one processing that is being discussed to improve the reception characteristic of an M-PDCCH, which is a PDCCH to transmit L1/L2 control information for MTC UEs, is to repeatedly transmit the M-PDCCH over multiple subframes. According to the above processing, communication characteristics of MTC UEs that are expected to be lower than those of normal UEs will be improved. When downlink data is scheduled by repetitive transmission of the M-PDCCH, it has been discussed to transmit this data at a subframe after the subframe at which the last repetitive transmission of the M-PDCCH is performed. It has further been discussed to include the number of repetitions of the M-PDCCH (the number of repetitions to be actually performed) in downlink (DL) control information contained in this M-PDCCH.
The number of repetitions of transmission and the number of repetitions of reception that are required for improvement of the communication characteristics depend on the place where am MTC UE is installed and the pathloss between the MTC UE and the eNB. Therefore, the coverage enhancement technique provides a plurality of coverage enhancement levels (CE levels). The coverage enhancement levels (CE levels) may also be referred to as enhanced coverage levels, coverage extension levels, extended coverage levels, or repetition levels (e.g., PRACH repetition levels). Further, a one-to-one relation or a certain relative relation may be configured in advance between the CE level and the Repetition level.
For example, the coverage enhancement technique provides three CE levels in addition to normal coverage (zero coverage extension). The CE levels are associated respectively with different numbers of repetitions of transmission and with different numbers of repetitions of reception. The number of repetitions of transmission and the number of repetitions of reception used in a high CE level are larger than those used in a low CE level. Each MTC UE is allocated to a higher CE level, as the pathloss between the MTC UE and the eNB increases. In some implementations, an MTC UE measures a Reference Signal Received Power (RSRP) from the eNB or measures an estimated pathloss between the MTC UE and the eNB, determines (or estimates) a required CE level based on the measured RSRP or pathloss, and then transmits a random access preamble (RACH preamble) in accordance with the maximum number of repetitions of transmission associated with the determined CE level (see Patent Literature 1).